There are many situations and applications where metal structures become subject to oxidative corrosion and ultimately fail to fulfill their intended purpose. Examples of failure by metal corrosion include deterioration of heat exchanger elements, corrosion of pipeline distribution systems and especially the gradual disintegration of steel used for reinforcing concrete structures such as bridge decks and frames which support a wide range of modern buildings.
Newly constructed metal structures typically have a protective treatment against corrosion. As the structures age, protection diminishes and corrosion processes occur. A deterrent to such processes would delay the onset of corrosion, especially if the deterrent exerted its effect later in the lifetime of the reinforced structures. Treatments to delay the onset of corrosion, as disclosed in subsequent prior art references, include application of corrosion inhibitors or protective coatings directly to the metal surface or release of protective agents into a matrix material.
U.S. Pat. No. 4,329,381 discloses a method for protecting metal surfaces with compositions containing a corrosion-protective amount of a five- or six-membered nitrogen-heterocyclic compound containing at least one zinc or lead salt and a suitable film-forming vehicle such as an alkyd or epoxy resin. Representatives of the heterocyclic compounds include hydantoin, 2-mercaptothiazole, 2-mercaptobenzoxazole and 2-mercaptobenzothiazole. U.S. Pat. No. 5,102,457 discloses anticorrosive surface coatings incorporating S-benzylated derivatives of 2-mercaptobenzothiazole as corrosion inhibitors. Epoxy resins, alkyd, acrylic, melamine, polyurethane or polyester resins or mixtures thereof provide suitable film-forming binders for the corrosion inhibitors disclosed.
U.S. Pat. No. 3,505,244 encapsulates a combination of corrosion inhibitor and anti-leak agents and thereafter processes the encapsulated material into a free-flowing powder. When added to cooling water, circulating in heat exchanger systems, the powder deters conditions associated with developing corrosion sites in the metal structure. Suitable corrosion inhibitors include benzotriazole, benzimidazole and derivatives and mixtures thereof. Encapsulated inhibitors may be introduced at any time during the life of a heat exchanger system.
Previous metal corrosion inhibiting compositions addressed the control or delay of corrosion either by incorporating dry inhibitor in a protective film applied to the metal surface or, as exemplified by U.S. Pat. No. 3,505,244, delivering inhibitor from capsules circulating in cooling water. The use of dry inhibitors, alone, is inefficient due to restrictions on the concentration of inhibitor for optimum coating properties and the inability of remedial material to migrate to the corrosion site. In the case of the encapsulated corrosion inhibitor, circulating in the water of heat exchangers, it appears that protective action occurs after the onset of damage when the exchanger has already sprung a leak.
U.S. Pat. No. 5,534,289 discloses a method for monitoring structures for crack formation, such formation to be indicated by color development at a crack site. The monitoring process involves using dye filled microcapsules in a coating applied to the structure. The microcapsules fracture under stress associated with crack formation causing a change of color near a crack. The discoloration will be noticeable during regular inspection of the structure, providing evidence of the need for maintenance personnel to take remedial action. Although they provide a warning of structural deterioration, neither the coating nor the dye containing microcapsules include agents suitable for preventing further damage to the structure. Damage control depends, therefore, on how frequently the structure is inspected.
U.S. Pat. Nos. 5,561,173, 5,575,841 and 5,660,624 disclose shaped structures. e.g., concrete blocks, using matrix reinforcing hollow fibers containing fluids suitable for effecting repair as the matrices age and deteriorate. In the matrix of concrete, the fluid containing hollow fibers provide reinforcement and a delivery means for repair of the concrete and associated structures subject to corrosive deterioration. The repair process releases anticorrosive fluids to cracks and other structural imperfections developed in the concrete by the action of stress, moisture and other corrosive components. Release of remedial fluids from hollow fibers causes distribution of protective chemical in the proximity of a damaged section but not necessarily at the precise location where remedial action is required due to the separation of the structure from the metal where damage may occur.
None of the cited prior art teaches how to re-seal scarred coatings and renew protection to an area of metal surface that became exposed by abrasion, impact or other conditions that cause disruption of a bonded protective coating. This situation is remedied by the present invention using a latent, film forming, corrosion protective fluid composition contained in rupturable microcapsules. Combined with a film forming binder, the microcapsules provide a thin corrosion protective coating for metal surfaces. The protective fluid composition, contained in microcapsules, provides precise delivers of metal corrosion protection in the immediate neighborhood of the damage site produced by abuse of the protective coating. In contrast, the fluid containing hollow fibers previously discussed, while fulfilling their matrix reinforcing function, do not provide immediate contact with internal metal structural reinforcing elements such as rebar due to the positioning of the hollow fibers in shaped structures such as blocks, involving thick sections which prevent immediate access to metal surfaces when compared with coatings of the present invention. Further, the hollow fibers are unsuitable for thin coatings; their dimensions will interfere with smooth and effective coating application over metal.
Free-flowing powder coatings of the invention provide improved metal protection using self-repairing compositions located within 200 .mu.m of the metal surface. Protective, microencapsulated components include anticorrosive chemicals, film forming components and marker dyes for visual identification of coating abuse or disruption.